illumos: manual page: gl_io

GL_IO_MODE(3TECLA) Interactive Command-line Input Library Functions

NAME

gl_io_mode, gl_raw_io, gl_normal_io, gl_tty_signals, gl_abandon_line,
gl_handle_signal, gl_pending_io - use gl_get_line() from an external
event loop

SYNOPSIS

cc [ flag... ] file... -ltecla [ library... ]
#include <libtecla.h>

int gl_io_mode(GetLine *gl, GlIOMode mode);

int gl_raw_io(GetLine *gl);

int gl_normal_io(GetLine *gl);

int gl_tty_signals(void (*term_handler)(int), void (*susp_handler)(int),
void (*cont_handler)(int), void (*size_handler)(int));

void gl_abandon_line(GetLine *gl);

void gl_handle_signal(int signo, GetLine *gl, int ngl);

GlPendingIO gl_pending_io(GetLine *gl);

DESCRIPTION

The gl_get_line(3TECLA) function supports two different I/O modes. These
are selected by calling the gl_io_mode() function. The mode argument of
gl_io_mode() specifies the new I/O mode and must be one of the following.

GL_NORMAL_MODE
Select the normal blocking-I/O mode. In this mode
gl_get_line() does not return until either an error
occurs of the user finishes entering a new line.

GL_SERVER_MODE
Select non-blocking server I/O mode. In this mode,
since non-blocking terminal I/O is used, the entry of
each new input line typically requires many calls to
gl_get_line() from an external I/O-driven event loop.

Newly created GetLine objects start in normal I/O mode, so to switch to
non-blocking server mode requires an initial call to gl_io_mode().

Server I/O Mode
In non-blocking server I/O mode, the application is required to have an
event loop that calls gl_get_line() whenever the terminal file descriptor
can perform the type of I/O that gl_get_line() is waiting for. To
determine which type of I/O gl_get_line() is waiting for, the application
calls the gl_pending_io() function. The return value is one of the
following two enumerated values.

GLP_READ
gl_get_line() is waiting to write a character to the
terminal.

GLP_WRITE
gl_get_line() is waiting to read a character from the
keyboard.

If the application is using either the select(3C) or poll(2) function to
watch for I/O on a group of file descriptors, then it should call the
gl_pending_io() function before each call to these functions to determine
which direction of I/O it should tell them to watch for, and configure
their arguments accordingly. In the case of the select() function, this
means using the FD_SET() macro to add the terminal file descriptor either
to the set of file descriptors to be watched for readability or the set
to be watched for writability.

As in normal I/O mode, the return value of gl_get_line() is either a
pointer to a completed input line or NULL. However, whereas in normal I/O
mode a NULL return value always means that an error occurred, in non-
blocking server mode, NULL is also returned when gl_get_line() cannot
read or write to the terminal without blocking. Thus in non-blocking
server mode, in order to determine when a NULL return value signifies
that an error occurred or not, it is necessary to call the
gl_return_status() function. If this function returns the enumerated
value GLR_BLOCKED, gl_get_line() is waiting for I/O and no error has
occurred.

When gl_get_line() returns NULL and gl_return_status() indicates that
this is due to blocked terminal I/O, the application should call
gl_get_line() again when the type of I/O reported by gl_pending_io()
becomes possible. The prompt, start_line and start_pos arguments of
gl_get_line() will be ignored on these calls. If you need to change the
prompt of the line that is currently being edited, you can call the
gl_replace_prompt(3TECLA) function between calls to gl_get_line().

Giving Up The Terminal

A complication that is unique to non-blocking server mode is that it
requires that the terminal be left in raw mode between calls to
gl_get_line(). If this were not the case, the external event loop would
not be able to detect individual key-presses, and the basic line editing
implemented by the terminal driver would clash with the editing provided
by gl_get_line(). When the terminal needs to be used for purposes other
than entering a new input line with gl_get_line(), it needs to be
restored to a usable state. In particular, whenever the process is
suspended or terminated, the terminal must be returned to a normal state.
If this is not done, then depending on the characteristics of the shell
that was used to invoke the program, the user could end up with a hung
terminal. To this end, the gl_normal_io() function is provided for
switching the terminal back to the state that it was in when raw mode was
last established.

The gl_normal_io() function first flushes any pending output to the
terminal, then moves the cursor to the start of the terminal line which
follows the end of the incompletely entered input line. At this point it
is safe to suspend or terminate the process, and it is safe for the
application to read and write to the terminal. To resume entry of the
input line, the application should call the gl_raw_io() function.

The gl_normal_io() function starts a new line, redisplays the partially
completed input line (if any), restores the cursor position within this
line to where it was when gl_normal_io() was called, then switches back
to raw, non-blocking terminal mode ready to continue entry of the input
line when gl_get_line() is next called.

Note that in non-blocking server mode, if gl_get_line() is called after a
call to gl_normal_io(), without an intervening call to gl_raw_io(),
gl_get_line() will call gl_raw_mode() itself, and the terminal will
remain in this mode when gl_get_line() returns.

Signal Handling

In the previous section it was pointed out that in non-blocking server
mode, the terminal must be restored to a sane state whenever a signal is
received that either suspends or terminates the process. In normal I/O
mode, this is done for you by gl_get_line(), but in non-blocking server
mode, since the terminal is left in raw mode between calls to
gl_get_line(), this signal handling has to be done by the application.
Since there are many signals that can suspend or terminate a process, as
well as other signals that are important to gl_get_line(), such as the
SIGWINCH signal, which tells it when the terminal size has changed, the
gl_tty_signals() function is provided for installing signal handlers for
all pertinent signals.

The gl_tty_signals() function uses gl_get_line()'s internal list of
signals to assign specified signal handlers to groups of signals. The
arguments of this function are as follows.

term_handler
This is the signal handler that is used to trap signals
that by default terminate any process that receives them
(for example, SIGINT or SIGTERM).

susp_handler
This is the signal handler that is used to trap signals
that by default suspend any process that receives them,
(for example, SIGTSTP or SIGTTOU).

cont_handler
This is the signal handler that is used to trap signals
that are usually sent when a process resumes after being
suspended (usually SIGCONT). Beware that there is nothing
to stop a user from sending one of these signals at other
times.

size_handler
This signal handler is used to trap signals that are sent
to processes when their controlling terminals are resized
by the user (for example, SIGWINCH).

These arguments can all be the same, if so desired, and SIG_IGN (ignore
this signal) or SIG_DFL (use the system-provided default signal handler)
can be specified instead of a function where pertinent. In particular, it
is rarely useful to trap SIGCONT, so the cont_handler argument will
usually be SIG_DFL or SIG_IGN.

The gl_tty_signals() function uses the POSIX sigaction(2) function to
install these signal handlers, and it is careful to use the sa_mask
member of each sigaction structure to ensure that only one of these
signals is ever delivered at a time. This guards against different
instances of these signal handlers from simultaneously trying to write to
common global data, such as a shared sigsetjmp(3C) buffer or a signal-
received flag. The signal handlers installed by this function should
call the gl_handle_signal().

The signo argument tells this function which signal it is being asked to
respond to, and the gl argument should be a pointer to the first element
of an array of ngl GetLine objects. If your application has only one of
these objects, pass its pointer as the gl argument and specify ngl as 1.

Depending on the signal that is being handled, this function does
different things.

Process termination signals

If the signal that was caught is one of those that by default terminates
any process that receives it, then gl_handle_signal() does the following
steps.

1. First it blocks the delivery of all signals that can be
blocked (ie. SIGKILL and SIGSTOP cannot be blocked).

2. Next it calls gl_normal_io() for each of the ngl GetLine
objects. Note that this does nothing to any of the GetLine
objects that are not currently in raw mode.

3. Next it sets the signal handler of the signal to its default,
process-termination disposition.

4. Next it re-sends the process the signal that was caught.

5. Finally it unblocks delivery of this signal, which results in
the process being terminated.

Process suspension signals

If the default disposition of the signal is to suspend the process, the
same steps are executed as for process termination signals, except that
when the process is later resumed, gl_handle_signal() continues, and does
the following steps.

1. It re-blocks delivery of the signal.

2. It reinstates the signal handler of the signal to the one that
was displaced when its default disposition was substituted.

3. For any of the GetLine objects that were in raw mode when
gl_handle_signal() was called, gl_handle_signal() then calls
gl_raw_io(), to resume entry of the input lines on those
terminals.

4. Finally, it restores the signal process mask to how it was
when gl_handle_signal() was called.

Note that the process is suspended or terminated using the original
signal that was caught, rather than using the uncatchable SIGSTOP and
SIGKILL signals. This is important, because when a process is suspended
or terminated, the parent of the process may wish to use the status value
returned by the wait system call to figure out which signal was
responsible. In particular, most shells use this information to print a
corresponding message to the terminal. Users would be rightly confused
if when their process received a SIGPIPE signal, the program responded by
sending itself a SIGKILL signal, and the shell then printed out the
provocative statement, "Killed!".

Interrupting The Event Loop

If a signal is caught and handled when the application's event loop is
waiting in select() or poll(), these functions will be aborted with errno
set to EINTR. When this happens the event loop should call
gl_pending_io() before calling select() or poll() again. It should then
arrange for select() or poll() to wait for the type of I/O that
gl_pending_io() reports. This is necessary because any signal handler
that calls gl_handle_signal() will frequently change the type of I/O that
gl_get_line() is waiting for.

If a signal arrives between the statements that configure the arguments
of select() or poll() and the calls to these functions, the signal will
not be seen by these functions, which will then not be aborted. If these
functions are waiting for keyboard input from the user when the signal is
received, and the signal handler arranges to redraw the input line to
accommodate a terminal resize or the resumption of the process. This
redisplay will be delayed until the user presses the next key. Apart from
puzzling the user, this clearly is not a serious problem. However there
is a way, albeit complicated, to completely avoid this race condition.
The following steps illustrate this.

1. Block all of the signals that gl_get_line() catches, by
passing the signal set returned by gl_list_signals() to
sigprocmask(2).

2. Call gl_pending_io() and set up the arguments of select() or
poll() accordingly.

3. Call sigsetjmp(3C) with a non-zero savemask argument.

4. Initially this sigsetjmp() statement will return zero,
indicating that control is not resuming there after a matching
call to siglongjmp(3C).

5. Replace all of the handlers of the signals that gl_get_line()
is configured to catch, with a signal handler that first
records the number of the signal that was caught, in a file-
scope variable, then calls siglongjmp() with a non-zero val
argument, to return execution to the above sigsetjmp()
statement. Registering these signal handlers can conveniently
be done using the gl_tty_signals() function.

6. Set the file-scope variable that the above signal handler uses
to record any signal that is caught to -1, so that we can
check whether a signal was caught by seeing if it contains a
valid signal number.

7. Now unblock the signals that were blocked in step 1. Any
signal that was received by the process in between step 1 and
now will now be delivered, and trigger our signal handler, as
will any signal that is received until we block these signals
again.

8. Now call select() or poll().

9. When select returns, again block the signals that were
unblocked in step 7.

If a signal is arrived any time during the above steps, our
signal handler will be triggered and cause control to return
to the sigsetjmp() statement, where this time, sigsetjmp()
will return non-zero, indicating that a signal was caught.
When this happens we simply skip the above block of
statements, and continue with the following statements, which
are executed regardless of whether or not a signal is caught.
Note that when sigsetjmp() returns, regardless of why it
returned, the process signal mask is returned to how it was
when sigsetjmp() was called. Thus the following statements are
always executed with all of our signals blocked.

10. Reinstate the signal handlers that were displaced in step 5.

11. Check whether a signal was caught, by checking the file-scope
variable that the signal handler records signal numbers in.

12. If a signal was caught, send this signal to the application
again and unblock only this signal so that it invokes the
signal handler which was just reinstated in step 10.

13. Unblock all of the signals that were blocked in step 7.

Signals Caught By gl_get_line()
Since the application is expected to handle signals in non-blocking
server mode, gl_get_line() does not attempt to duplicate this when it is
being called. If one of the signals that it is configured to catch is
sent to the application while gl_get_line() is being called,
gl_get_line() reinstates the caller's signal handlers, then immediately
before returning, re-sends the signal to the process to let the
application's signal handler handle it. If the process is not terminated
by this signal, gl_get_line() returns NULL, and a following call to
gl_return_status() returns the enumerated value GLR_SIGNAL.

Aborting Line Input

Often, rather than letting it terminate the process, applications respond
to the SIGINT user-interrupt signal by aborting the current input line.
This can be accomplished in non-blocking server-I/O mode by not calling
gl_handle_signal() when this signal is caught, but by calling instead the
gl_abandon_line() function. This function arranges that when
gl_get_line() is next called, it first flushes any pending output to the
terminal, discards the current input line, outputs a new prompt on the
next line, and finally starts accepting input of a new input line from
the user.

Signal Safe Functions

Provided that certain rules are followed, the gl_normal_io(),
gl_raw_io(), gl_handle_signal(), and gl_abandon_line() functions can be
written to be safely callable from signal handlers. Other functions in
this library should not be called from signal handlers. For this to be
true, all signal handlers that call these functions must be registered in
such a way that only one instance of any one of them can be running at
one time. The way to do this is to use the POSIX sigaction() function to
register all signal handlers, and when doing this, use the sa_mask member
of the corresponding sigaction structure to indicate that all of the
signals whose handlers invoke the above functions should be blocked when
the current signal is being handled. This prevents two signal handlers
from operating on a GetLine object at the same time.

To prevent signal handlers from accessing a GetLine object while
gl_get_line() or any of its associated public functions are operating on
it, all public functions associated with gl_get_line(), including
gl_get_line() itself, temporarily block the delivery of signals when they
are accessing GetLine objects. Beware that the only signals that they
block are the signals that gl_get_line() is currently configured to
catch, so be sure that if you call any of the above functions from signal
handlers, that the signals that these handlers are assigned to are
configured to be caught by gl_get_line(). See gl_trap_signal(3TECLA).

Using Timeouts To Poll

If instead of using select() or poll() to wait for I/O your application
needs only to get out of gl_get_line() periodically to briefly do
something else before returning to accept input from the user, use the
gl_inactivity_timeout(3TECLA) function in non-blocking server mode to
specify that a callback function that returns GLTO_CONTINUE should be
called whenever gl_get_line() has been waiting for I/O for more than a
specified amount of time. When this callback is triggered, gl_get_line()
will return NULL and a following call to gl_return_status() will return
GLR_BLOCKED.

The gl_get_line() function will not return until the user has not typed a
key for the specified interval, so if the interval is long and the user
keeps typing, gl_get_line() might not return for a while. There is no
guarantee that it will return in the time specified.

ATTRIBUTES